1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus including plural image creating units, in which an image creating mechanism is driven by plural electric motors. This image forming apparatus can be used as a printer, a copier, and a facsimile machine.
2. Description of the Related Art
There are image forming apparatuses that include plural image creating units, such as a tandem color copier in which a black image creating unit, a magenta image creating unit, a cyan image creating unit, and a yellow image creating unit are arranged along an intermediate transfer belt in this order from the upstream side of the movement direction of the intermediate transfer belt. Because a large machine load needs to be driven in such an image forming apparatus, plural electric motors are often employed (see, for example, patent documents 1 and 2). In order to reduce noise emitted when the electric motors drive the mechanism and to reduce power consumption, there has been proposed a technology for driving the image creating units and the intermediate transfer belt with separate DC brushless motors (see patent document 2).
However, even in a case where the operation of driving the machine load is divided among plural electric motors, the following problem occurs. Because the startup current value of each electric motor is high, if all of the electric motors are simultaneously started up at substantially the same time as the image forming apparatus is started up, the load current for starting up the apparatus will be extremely high. Such an extremely high load may be reported to the power source as an abnormal load, and the power source may be automatically shut off. In order to avoid such a situation and to start up the apparatus in a quick and efficient manner, the startup timings of the electric motors are shifted from each other, so that the startup currents, which would otherwise coincide with each other, are chronologically distributed. For example, patent document 1 discloses a startup control method of measuring the startup current value of each electric motor, saving the measured values, and determining the individual startup timings of each electric motor.
However, because the startup current of each electric motor needs to be measured individually, it is time consuming to measure all of the electric motors. Furthermore, there is a need to provide a special current detecting circuit for detecting a startup current that fluctuates at high speed, which leads to increased cost. Moreover, the electric motor load in an electrostatic image forming apparatus frequently fluctuates. For example, the temperature inside the image forming apparatus fluctuates over a wide range, and the machine load fluctuates according to the temperature. The load fluctuation caused by the temperature fluctuation causes fluctuations in the frictional resistance as the motion mechanism expands and contracts. Furthermore, in an electrophotographic image forming apparatus for forming an electrostatic latent image and developing the image with toner, the driving load of the developing unit fluctuates considerably. The fluidity of toner fluctuates due to fluctuations in the temperature and humidity, and the rotational load of a conveying/stirring screw fluctuates due to fluctuations in the remaining amount of toner in the developing unit. Even though such fluctuations occur, it is necessary to constantly prevent the startup currents of different electric motors from coinciding with each other, so that the total load does not become excessively high. Accordingly, the startup currents of the electric motors need to be frequently measured individually, and the measurement data in the memory needs to be frequently rewritten. If there are long time intervals between operations of individually measuring the startup currents of the electric motors and rewriting the memory data, the current values in the memory will not reflect load fluctuations caused by fluctuations in the remaining amount of toner, temperature, humidity, and the like. As a result, the startup load cannot be reliably distributed. It is necessary to determine the appropriate timings for measuring the startup currents and rewriting the memory data.
Patent Document 1: Japanese Laid-Open Patent Application No. 2004-138840
Patent Document 2: Japanese Laid-Open Patent Application No. 2005-300868